Rock drill bit and cutting inserts

ABSTRACT

The present invention relates to a cutting insert for a rock drill bit and a rock drill bit including such a cutting insert. It has the object of increasing the wear resistance of the cemented carbide cutting insert. The inserts are formed with a generally cylindrical shank portion and a convexly formed outer portion. In one embodiment of the invention, the cemented carbide of the insert includes a number of zones and the border between two adjacent zones describes a non-symmetrical path seen both in a cross-sectional side view and in a cross-sectional top view. In a further embodiment, the inserts are also provided with increased volume portions in the parts of the insert being most subjected to wear.

BACKGROUND OF THE INVENTION

The present invention relates to inserts of cemented carbide bodies and rock drill bits preferably for percussive rock drilling.

In U.S. Pat. No. 4,598,779 is shown a rock drill bit that is provided with a plurality of chisel-shaped cutting inserts. Each insert discloses a guiding surface that is relatively sharply connected to cutting edges. A relatively sharp connection is disadvantageous when using cemented carbide that is extra hard. That is, flaking will occur during severe rock drilling due to tension in the connections, such that straight holes may not be achieved in the long run. Also the shape of the known insert is not optimized for maximum wear volume. U.S. Pat. No. 4,607,712 discloses a rock drill bit which has a plurality of cutting inserts. The working part of each insert has a semispherical basic shape, to which has been added extra volume of cemented carbide. However, the prior art insert does not sufficiently support against the wall of the bore such that straight holes may not be achieved. Furthermore, connections between the components of the working part are relatively sharp thereby producing the above-mentioned tensions detrimental for hard cemented carbide. In addition, the spherical basic shape holds a relatively small volume of cemented carbide.

Cemented carbide for rock drilling purposes generally contain WC, often referred to as alfa phase, and binder phase, which consists of cobalt with small amounts of W and C in solid solution, referred to as beta-phase. Free carbon or eta-phases, low carbon phases with the general formulas M₆ C (CO₃ W₃ C), M₁₂ C (Co₆ W₆ C) or kappa-phase M₄ C are generally not present. However in EP-B2-0 182 759 cemented carbide bodies are disclosed with a core of fine and evenly distributed eta-phase embedded in the normal alpha+beta-phase structure, and a surrounding surface zone with only alpha+beta-phase. An additional condition is that in the inner part of the surface zone situated close to the core the binder phase content is higher than the nominal content of binder phase. In addition the binder phase content of the outermost part of the surface zone is lower than the nominal and increases in the direction towards the core up to a maximum situated in the zone free of eta-phase. With nominal binder phase content is meant here and henceforth weighed-in amount of binder phase.

In U.S. Pat. No. 5,286,549 cemented carbide bodies are disclosed, comprising WC(alpha-phase) and a binder phase based on at least one of Co, Fe and Ni and comprising a core of eta-phase-containing cemented carbide surrounded by a surface zone with an outer part of the surface zone having a lower binder phase content than the nominal, the binder phase content in the outer part of the surface zone being substantially constant. Cemented carbide bodies produced according to this invention have a high wear resistance because of a higher average hardness in the outer zone. Other related documents are U.S. Pat. No. 5,279,901 and EP-A-92850260.8. Cemented carbide bodies with a structure similar to EP-B2-0 182 759 are useful also as a punching or nibbling tool material as disclosed in U.S. Pat. No. 5,235,879 or as a roll material as in EP-A-93850023.8. Furthermore the material disclosed in U.S. Pat. No. 5,074,623 could also be used.

The object of the latter seven inventions (which are incorporated with the description by reference) is to achieve high wear resistance at the outer zone caused by the high hardness in combination with compressive pre-stresses caused by the different binder contents in the different zones. If the wear flat which develops during wear reaches the zone having a binder content higher than the nominal, the wear resistance is decreasing rapidly because of the lower hardness. This has been an disadvantage, in particular in rock drilling with insert-equipped bits.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to avoid or alleviate the problems of the prior art. One object of the invention is to increase the wear resistance of cemented carbide bodies preferably for use in tools for rock drilling and mineral drilling, by adaption of the design of the cemented carbide body to the specific demands of cemented carbide produced in accordance with prior art. The wear resistance of the cemented carbide body can be increased by increasing the body volume in the area exposed to wear. In order to reach a distinct increase of the wear resistance, the volume of the outer zone exposed to wear has to be increased essentially. It has now surprisingly turned out that it is possible to increase the wear resistance of cemented carbide bodies having an outer zone with low binder content (high hardness/high wear resistance), a zone between the outer zone and the core with high binder content (low hardness/low wear resistance) and a core containing eta-phase by increasing the volume of the area outer zone where the wear occurs. A distinct increase of the wear resistance can be obtained when increasing the volume of the outer zone which is exposed to wear when the tool is in operation by at least 50%, probably 100% or more. Inserts in percussive drill bits wear most in the area which comes in contact with a hole wall and in the top of the insert where the rock has to be broken. In order to increase the wear resistance of an insert with an outer zone which has lower binder content than the nominal binder content, the volume of the outer zone has to be increased in the area coming in contact with the wall and in the top. Prior art tools normally have inserts with an axial-symmetric top design (left part of FIG. 12). An increase of the outer zone which is exposed to wear often leads to a non-axial symmetric top. Due to the nature of the wear, which depends on the rock properties and the drilling conditions, the wear appears pronounced in the area coming in contact with the wall or in the top area where the rock is broken. It is important to respect this fact and increase the volume of the outer zone most where the inserts wear most.

Both longer life and higher penetration rate can be achieved because the optimal structure will not be destroyed as fast. An important advantage of the invention is a higher precision when using the material in drill bits. The high wear resistance of the outer zone and the enlargened volume of wear resistant material in the area exposed to wear gives much better diameter tolerances of the drilled hole.

The objects of the present invention are realized by an insert and a rock drill bit that has been given the characteristics of the appending claims.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1-5 show an insert suitable to drill under conditions where the wear of the insert is concentrated in the area close to the wall. FIG. 1 shows an insert according to the present invention, in a side view. FIG. 2 shows the insert in another side view. FIG. 3 shows the insert in a top view. FIG. 4 shows the insert in a view according to arrow B in FIG. 2. FIG. 5 shows an enlarged cross-section of the insert as seen at line C.

FIGS. 6-10 show an insert suitable to drill under conditions where the wear of the insert is distributed in the area close to the wall and in the top area. FIG. 6 shows an insert according to the present invention, in a side view. FIG. 7 shows the insert in another side view. FIG. 8 shows the insert in a top view. FIG. 9 shows the insert in a view according to arrow B in FIG. 7. FIG. 10 shows an enlarged cross-section of the insert as seen at line C'.

FIG. 11 shows a drill head according to the present invention, in a perspective view.

FIG. 12 shows a side view, partly in section, of a schematically illustrated drill head with a ballistic insert and an insert according to the present invention, in a bore hole.

FIGS. 13 to 18 show cross-sectional views through the center axes of the two cutting inserts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows an enlarged side view of a preferred embodiment of an insert according to the present invention. The insert has a generally cylindrical shank portion 20 having a diameter D within the interval 4 to 20 mm, preferably 7 to 18 mm. The mounting end 21 of the insert 14 has preferably a frusto-conical shape adapted to enter into a hole in the drill head front surface, see FIG. 11. Preferably, the hole emerges both in the front surface as well as the jacket surface. In the figures the longitudinal center axis A of the insert and two right-angled normals N1 and N2 are shown. A line Y is defined as the base of the working part 22. The line may be distinct or smooth.

The working part 22 of the insert 14 is divided into seven smoothly connecting substantially circumferentially and axially convex portions. By the expression "smooth" or "smoothly" is hereinafter meant that two tangents, perpendicular to the center axis A in side view, each disposed on separate sides in the immediate vicinity of the connection, form an angle T which is in the interval of 135° to 180°, preferably 160° to 175° (FIG. 5). A first portion 23 describes a generally ballistic shape and extends generally symmetrically on both sides of the normal N1. The first portion ends circumferentially at symmetrically disposed radius zone lines 24 and 25, respectively. The radius of the first portion in a certain axial cross-section C is designated R1. The mathematical construction of the ballistic shape is as follows:

The reference plane X of the first portion 23 lies beneath the base line Y in FIG. 2. The convex curvature of the first portion 23 is struck from the radii R with a center Z in the vicinity of the envelope surface of the shank portion 20. The center Z is preferably placed outside the envelope surface a distance I and below the axially forwardmost point a distance h. The distance h is 4 to 8 times the distance I but smaller than the radius R. The reference plane X and the radii R enclose an angle E between 10° and 75°.

Each radius zone line 24 and 25, respectively, and the normal N1, seen in a top view, enclose an angle α within the interval of 45° to 85°. It is understood that the ballistic convex curvature radially outermost is connected to the envelope surface of the shank portion 20.

The radius zone line 24 or 25 represents a smooth transition between the first portion 23 and a second portion 26 or 27. The second portion 26 or 27 is except for the immediate junction with the first portion, disposed generally outside the ballistic basic shape (drawn with broken lines in FIGS. 1, 2 and 4). The radius R2 of the second portion in the cross-section C is larger than the radius R1 of the first portion. The second portion substantially tapers in the forward direction of the centre axis A. The second portions 26, 27 taper towards the first portion 23 and form an acute angle β.

The second portion 26 or 27 further connects to a third portion 28 or 29. The third portions merge radially off the axis A at the front portion of the insert. The third portions are crestlike strong edges that machine the rock mainly in the circumferential direction. A tangent of the third portion at the intersection of cross-section C is at larger internal angle φ1 with respect to the envelope surface of the shank portion than are corresponding tangents of the first and second portions. The magnitude of angle φ1 causes an increase in material to wear in comparison with an entire ballistic configuration and thus increases the wear resistance of the insert. The third portion is defined by a radius R3 which is smaller than both the radius R1 of the first portion and the radius R2 of the second portion in the cross-section C (see FIG. 5). The width of the third portion is substantially constant.

The third portion smoothly connects to a fourth portion 30 which is adapted to mainly coincide with and lie mainly flush with the wall of the drilled hole. The fourth portion defines a guiding surface provided to slide on the wall of the bore. The fourth portion has a radius R4 in the cross-section C, which is much larger than each of the above-mentioned radii R1 and R3. A central tangent of the portion 30 in the cross-section C-C forms an internal angle φ relative to the envelope surface of the shank 20. The angle φ is smaller than corresponding angles of each of the other portions 23-27.

A first part of the base line Y connected to the first portion 23, extends substantially perpendicular to the center axis A. A second part of the base line Y connected to the second portion 24 or 25, rises at least partially, forwardly at an acute angle δ relative to the first part. A third part of the base line Y connected to the third portion 28 or 29, discloses the axially forwardmost point of the entire base line and is generally defined by a radius R6. The third part is convex. A fourth part of the base line Y connected to the fourth portion 30, is generally defined by a radius R5 larger than the radius R6. The fourth part is concave and its rearwardmost point lies axially forwards of the first part.

The fifth portion 31 is a rounded apex wherein the portions 23, 24, 25, 26 and 27 merge. The fourth portion 30 ends axially rearwardly of the apex 31. The axially forwardmost part of the third portion 28 or 29 is mainly not a part of the apex although it is connected thereto.

It should be noted that at the base line Y, above-mentioned radii R1, R2, R3 and R4 in a top view projection, are equal, i.e., equal to D/2.

Under certain mining conditions drill inserts may be more worn on one side than on the other and therefore it was developed an insert for use under such conditions, i.e., an insert with a bulk of material disposed asymmetrically with respect to the normal N1. That is, the bulk is disposed on the windward side and an increased clearance surface on the leeward side of the normal N1. FIG. 6 shows an enlarged side view of a preferred embodiment of an insert according to the present invention. The insert has a generally cylindrical shank portion 20' having a diameter D within the interval 4 to 20 mm, preferably 7 to 18 mm. The mounting end 21' of the insert 14' has preferably a frusto-conical shape adapted to enter into a hole (not shown) in the drill head front surface. Preferably, the hole emerges both in the front surface as well as the jacket surface. In the figures the longitudinal center axis A of the insert and two right-angled normals N1 and N2 are shown. A line Y' is defined as the base of the working part 22'.

The working part 22' of the insert 14' is divided into a number of smoothly connecting substantially circumferentially and axially convex portions. A first portion 23' describes a generally ballistic shape and extends asymmetrically on both sides of the normal N1. The first portion ends circumferentially at asymmetrically disposed radius zone lines 24' and 25', respectively. The radius of the first portion in a certain axial cross-section C' is designated R1. The mathematical construction of the ballistic shape has been discussed above.

The radius zone line 24' or 25' represents a smooth transition between the first portion 23' and second portions 26' and 27'. The second portion 26' consists of three smoothly connected parts. A first part 26'A of the second portion 26' and the second portion 27' are except for the immediate junction with the first portion disposed generally outside the ballistic basic shape (drawn with broken lines in FIGS. 6, 7 and 10) and is generally perpendicular with each other in the cross-section C'. The radius of the first part 26'A and the second portion 27' in the section C' is larger than the radius R'1 of the first portion and is in the same magnitude as the above-mentioned radius R2. The first part 26'A and the second portion 27' substantially tapers in the axially forward direction of the centre axis A and form an angle β', generally perpendicular in cross-section C'. A second part 26'B of the second portion 26' is disposed radially outside the ballistic basic shape. The radius R'2B of the second part in the cross-section C is larger than the radius R'1 of the first portion but smaller than the radius R2. The second part substantially tapers in the forward direction of the centre axis A.

A third part 26'C of the second portion 26' is also disposed radially outside the ballistic basic shape on the windward side W of the normal N1 of the insert. The radius R'2C of the third part in the cross-section C' is larger than the radius R'1 of the first portion. The third part substantially tapers in the forward direction of the centre axis A. The windward side W is the part of the insert that wears the most during machining of the rock material.

The third part 26'C and the second portion 27' further connects to third portions 28' and 29', respectively. The third portions merge radially off the axis A at the front portion of the insert 14'. The third portion 29' is much larger, at least 2 times larger, than the portion 28'. A tangent of the third aims portion 28' at the intersection of cross-section C' is at larger internal angle φ'1 with respect to the envelope surface of the shank portion than are corresponding tangents of the first portion 23' and the third portion 29'. The angle φ'1 giving rise to an further increase in material to wear in comparison with an entire ballistic configuration and thus increases the wear resistance of the insert. The third portion 29' is formed on the leeward side L of the normal N1 is defined by a radius R'3 which is smaller than both the radius R'1 of the first portion and the radius R'2 of the second portion in the cross-section C' (see FIG. 10). The width of the third portion 28' is substantially constant while the portion 29' tapers considerably axially forwards. The third portion 29' defines a strong crest like cutting edge.

The third portions 28' and 29' smoothly connects to a fourth portion 30' which is adapted to mainly coincide with and lie mainly flush with the wall of the drilled hole. The fourth portion defines a guiding surface provided to slide on the wall. The fourth portion has a radius R'4 in the cross-section C, which is much larger than each of the above-mentioned radii R'1 and R'3. A central tangent of the portion 30' forms an internal angle φ' relative to the envelope surface of the shank 20 in the cross-section C'. The angle φ' is smaller than corresponding angles of each of the other portions 23'-27'.

A first part of the base line Y' connected to the first portion 23', extends substantially perpendicular to the center axis A. A second part of the base line Y' connected to the portions 26'A and 27', rises at least partially, forwardly at an acute angle δ' relative to the first part. Third parts of the base line Y' connected to the third part 26' C and the third portion 29', disclose the axially forwardmost point of the entire base line. One of the third parts of the base line in connection with the third portion 29' is convex in a side view, while the other third part connected to the third part 26'C is mainly straight. A fourth part of the base line Y' connected to the fourth portion 30', is generally defined by a radius R'5 (in a side view) which is about the same as radius R'1. The fourth part is concave and its rearwardmost point lies axially forwards of the first part.

The fifth portion 31' is a rounded apex wherein the portions 23',26'A,26'B,26'C and 27' merge. The fourth portion 30' ends axially rearwardly of the apex 31'. The axially forwardmost part of the third portion 28 or 29 is mainly not a part of the apex although it is connected thereto.

It should be noted that at the base line Y' the above-mentioned radii R'1,R'2B,R'2C,R'3 and R'4 in a top view projection, are equal, i.e., equal to D/2.

In the embodiment shown in a perspective view in FIG. 11, the improved rock drill bit of the impact type is generally designated 10 and has a drill head 11, a shaft 12, a front end including a front surface 13 provided with a plurality of fixed carbide inserts 14 or 14'. The jacket surface 16 of the rock drill bit 10 has a cylindrical or frusto-conical shape, and is defined in FIG. 11 at the drill head. The jacket surface is defined at the largest diameter of steel part of the drill bit body. The inserts 14, 14' are inserted into holes in the drill bit body so that their radially outermost surfaces 30, 30' substantially coincide with the jacket surface of the drill bit. It is understood that the word "substantially" in this context includes a radial displacement of -2 to +2 mm relative to the jacket surface 16 of the drill bit, preferably +0.2 to +0.5 mm. The inserts 14, 14' are arranged such that the steel body will not be excessively worn and therefore the diameter of the bore 15 remains substantially constant during the entire drilling operation. The front surface 13 may have a number of more centrally placed inserts (not shown) of appropriate shape, for example semi-spherical shape, the latter inserts cracking rock material closer to the center line CL of the drill bit. In FIG. 12 are shown a prior art solution to the left and an insert according to the present invention to the right, partly in cross-section. An insert with a ballistic working part has a volume that is 50% greater than a corresponding semispherical working part. The volume of the insert 14 or 14' is at least 50% greater than the ballistic shape and has a life which is in parity therewith. In FIG. 12 an imaginary extension of the jacket surface 16 is drawn with broken lines so as to illustrate differences in volume of the two inserts.

In order to handle the high tensile stresses arising during rock drilling it is preferable to use a special type of cemented carbide disclosed in the above discussed seven patent documents. Therefore these publications are included in this specification by way of reference.

Referring now to FIGS. 13 to 18, the cemented carbide of the cutting insert 14 or 14' includes a number of zones H, I and K. Borders 50, 51 and 50', 51', respectively, of adjacent zones describe paths which are non-symmetrical, in at least one cross-sectional side view, with respect to the center axis A. The path in a cross-sectional top view is non-symmetrical with respect to at least one axis N2 perpendicular to the center axis. The insert has a core H of cemented carbide containing eta-phase. The core H is surrounded by an intermediate layer I of cemented carbide free of eta-phase and having a high content of cobalt. The surface layer K consists of cemented carbide free from eta-phase and having a low content of cobalt. The thickness of the surface layer is 0,8-4, preferably 1-3, of the thickness of the intermediate layer. The paths 50, 50' and 51, 51', respectively are preferably equidistant.

The core and the intermediate, cobalt rich layer have high thermal expansion compared to the surface layer. This means that the surface layer will be subjected to high compressive stresses. The bigger the difference in thermal expansivity, i.e. the bigger the difference in cobalt content between the surface layer and the rest of the cutting insert, the higher the compressive stresses in the surface layer. The content of binder phase in the surface layer is 0,1-0,9, preferably 0,2-0,7, of the nominal content of binder phase for the cutting insert 14 or 14'. The content of binder phase in the intermediate layer 16 is 1,2-3, preferably 1,4-2,5, of the nominal content of binder phase for the cutting insert 14 or 14'.

The insert 14 or 14' can be made of cemented carbide as disclosed in EP-A-0182759 wherein cemented carbide bodies are disclosed with a core H of fine and evenly distributed eta-phase embedded in the normal alpha+beta-phase structure I, and a surrounding surface zone K with only alpha+beta-phase. An additional condition is that in the inner part of the surface zone situated close to the core the binder phase content is higher than the nominal content of binder phase. In addition the binder phase content of the outermost part of the surface zone is lower than the nominal and increases in the direction towards the core up to a maximum situated in the zone free of eta-phase.

Alternatively the insert 14 or 14' can be made of cemented carbide as disclosed in U.S. Pat. No. 5,286,549 wherein cemented carbide bodies are disclosed, comprising WC(alpha-phase) and a binder phase based on at least one of Co, Fe and Ni and comprising a core of eta-phase-containing cemented carbide surrounded by a surface zone with an outer part of the surface zone having a lower binder phase content than the nominal, the binder phase content in the outer part of the surface zone being substantially constant.

From what is said above it can be realized that a higher nominal cobalt content of the cutting insert gives higher compressive stresses in the surface layer.

EXAMPLE 1

A test with 45 mm drifter drilling bits was performed in Norway (Tunnelling). The bits had 5 periphery inserts with a diameter of 11 mm and two front inserts with a diameter of 8 mm. The front inserts of all variants were made of conventional cemented carbide and had the same design with a semi-spherical top.

Variant 1 was a conventional bit with inserts having spherical top. The inserts were made of conventional cemented carbide (6 weight % Co, hardness 1460 HV3).

Variant 2 was a conventional bit with inserts having a spherical top. The inserts were made with an outer zone having low Co-content (3 weight % Co, hardness 1620 HV3), an intermediate zone having high Co-content (11 weight % Co, hardness 1240 HV3) and a core containing 6 weight % Co and some eta-phase, hardness 1550 HV3).

Variant 3 was a bit having inserts according to the present invention (FIGS. 1-4) and the same distribution of Co and properties as said in variant 2.

Test data

Drilling rig: Atlas Copco Promec TH 506S

Feeding pressure: 110 bar

Impact pressure 215 bar

Rotation: 120 rpm

Hole depth: 4.3 m

Water flushing: 11 bar

Rock: Gneiss

Number of bits: 6 per variant

Test results

All bits were drilled without regrinding and with regard to the users demand.

    ______________________________________                     Penetration rate                                 Diam wear     Variant            Drilled m                     (m/min)     (Drill m/mm)                                           Index*     ______________________________________     1      256      1, 4         90       100     2      322      1, 6        120       126     3      398      2, 1        164       155     ______________________________________      *Index for drilled m

Besides the excellent life time for variant 3 it showed a much lower hole diameter deviation because of the high diameter wear resistance. The high penetration rate of variant 3 is important for the drilling economy.

EXAMPLE 2

The purpose with the test was to be able to complete one hole, 60 m deep without resharpening. The standard bits today have to be sharpened after only 24 m because of slow drilling rate and risk for button and bit breakage. The down time of pulling out rods, changing bits and to continue to drill is approximately one hour. As the effective working time in this mine for each shift is only 6 hours the demand of better bits is very high.

Test data

Drill rig: XL 5,5 hammer air pressure 25 bar, mine air and booster compressor 280 bar

Rock: Very hard and abrasive, about 80% Silica, about 8% Pyrite

Drill hole dimension: Diameter 115 mm, hole depth 65 m

Rotation speed: 40 rpm

Number of bits: 4 per variant

Bit: Diameter 115 mm, 2 flushing holes, 8 inserts (16 mm diameter) on the periphery, 6 inserts (14 mm) on the front

Variants

A: Inserts with spherical top. All inserts made of conventional cemented carbide.

B: Ballistic inserts. All inserts made with an outer zone with low Co-content (3 weight % Co, hardness 1650 HV3), an intermediate zone with high Co-content (10.5 weight % Co, hardness 1260 HV3) and a core with 6 weight % Co, (hardness 1570 HV3). All other inserts made of conventional cemented carbide (6,0 weight % Co, hardness 1450 HV3).

C: In the front ballistic inserts, on the periphery inserts according to the present invention (FIGS. 6-9). All inserts made of cemented carbide as described under variant B.

Test results

All bits have been tested without regrinding.

    ______________________________________     Variant            Drilled m Penetration rate m/min                                     Index, drilled m     ______________________________________     A      28        0, 3           100     B      46        0, 35          164     C       62*      0, 45          221     ______________________________________      *length of the hole

Variant B performed much better than A but not enough. Only with variant C it was possible to drill a complete hole.

It should be pointed out that the core of cemented carbide containing eta-phase is stiff, hard and wear resistant. The core H in combination with an intermediate layer free of eta-phase and having a high content of cobalt and a surface layer free of eta-phase and subjected to high compressive stresses presents a cutting insert 14 or 14' that fulfills the requirements discussed above for drilling of hard stone, i.e. an insert having a high wear resistance especially in connection with cutting inserts according to the present invention. The core H has a binder phase content in the interval 4 to 9%, preferably about 6%; the intermediate layer I has a binder phase content of 9.5 to 20%, preferably about 10 to 11% and the surface zone K has a binder phase content of 0.5 to 3.9%, preferably about 3%.

In this connection it should be pointed out that the invention described above is not limited to the preferred embodiments but can be varied freely within the scope of the appending claims. For instance when the rock to be drilled is extremely hard (e.g. cracked and lamellar magnetite+quartzite rock) it will be necessary to reduce the height between the apex and the base line Y, Y' thereby increasing the average thickness of the working part 22, 22' and thus increasing wear resistance. Such modification would render the ballistic surfaces 23, 23' to assume a generally spherical shape. 

We claim:
 1. A cutting insert of cemented carbide preferably for percussive drilling comprising a generally cylindrical mounting portion and an outer portion extending from said mounting portion toward a forward end of the cutting insert, said cutting insert configured to be arranged at a front end of a rock drill bit, said outer portion including a relatively flat surface extending from said mounting portion towards the forward end of said insert, said mounting portion having a center axis, said mounting portion having a radius, wherein the cutting insert includes a number of zones, one of which is a surface zone completely surrounding a core zone of the cutting insert, and a border between two adjacent zones defines a path which is non-symmetrical, in at least one cross-sectional side view, with respect to the center axis, the path in a cross-sectional top view is non-symmetrical with respect to at least one axis perpendicular to the center axis.
 2. A cutting insert according to claim 1, wherein the outer portion is generally in the form of a convexly curved ballistic shape, and the relatively flat surface smoothly transitions into adjacent regions of said outer portion.
 3. A cutting insert according to claim 1, wherein the outer portion has a generally ballistic shape and the relatively flat surface has a radius of curvature which is larger than a radius of the cylindrical mounting portion, and the relatively flat surface is circumferentially connected to at least one crestlike cutting edge.
 4. A cutting insert according to claim 1 wherein a junction between the mounting portion, the outer portion and the relatively flat surface forms a concave base line, as seen in a side view, the concave base line defining an axially rearwardmost point, said rearward most point is disposed axially forward of the base line at the convexly curved basic shape but axially rearward of an axially forwardmost part of the base line.
 5. A cutting insert according to claim 1, wherein the core comprises a microstructure having a fine and evenly distributed eta-phase embedded in a normal alpha+beta-phase structure, and the surrounding surface zone comprises a microstructure having an alpha+beta-phase essentially free of any eta-phase, an inner part of the surface zone in close proximity to the core zone having a binder phase content which is higher than a nominal content of binder phase for the cutting insert, a binder phase content of an outermost part of the surface zone which is lower than the nominal content and increases in a direction towards the core and reaches a maximum in the surrounding surface zone which is essentially free of eta-phase.
 6. A cutting insert according to claim 1, wherein the insert comprises alpha-phase tungsten carbide and a binder phase having at least one of Co, Fe and Ni the core zone having a microstructure of eta-phase-containing cemented carbide surrounded by the surface zone, with an outer part of the surface zone having a lower binder phase content than a nominal binder phase content of the cutting insert, the binder phase content in the outer part of the surface zone being substantially constant.
 7. A rock drill bit of the impact type comprising a shaft, a boring head situated at a forward end of said shaft and defining a first longitudinal axis, said boring head comprising a generally forwardly facing front end including a front surface, a jacket surface extending generally longitudinally and defining the outer periphery of said boring head, and a plurality of holes formed in said front end, said holes each being generally cylindrical for holding a cemented carbide cutting insert therein, each insert comprising a generally cylindrical mounting portion having a center axis and an outer portion extending from said mounting portion toward a forward end of the cutting insert and extending out of said hole, wherein the cutting insert includes a number of zones, one of which is a surface zone completely surrounding a core zone of the insert and that a border between two adjacent zones defines a path which is non-symmetrical, in a cross-sectional side view, with respect to the center axis and that the path in a cross-sectional top view is non-symmetrical with respect to at least one axis perpendicular to the center axis.
 8. A cutting insert according to claim 7, wherein the outer portion is generally in the form of a convexly curved ballistic shape, and the relatively flat surface smoothly transitions into adjacent regions of said outer portion.
 9. A cutting insert according to claim 7, wherein the outer portion has a generally ballistic shape and the relatively flat surface has a radius of curvature which is larger than a radius of the cylindrical mounting portion, and the relatively flat surface is circumferentially connected to at least one crestlike cutting edge.
 10. A cutting insert according to claim 7, wherein the core comprises a microstructure having a fine and evenly distributed eta-phase embedded in a normal alpha+beta-phase structure, and the surrounding surface zone comprises a microstructure having an alpha+beta-phase essentially free of any eta-phase, an inner part of the surface zone in close proximity to the core zone having a binder phase content which is higher than a nominal content of binder phase for the cutting insert, a binder phase content of an outermost part of the surface zone which is lower than the nominal content and increases in a direction towards the core and reaches a maximum in the surrounding surface zone which is essentially free of eta-phase.
 11. A cutting insert according to claim 7, wherein the insert comprises alpha-phase tungsten carbide and a binder phase having at least one of Co, Fe and Ni the core zone having a microstructure of eta-phase-containing cemented carbide surrounded by the surface zone, with an outer part of the surface zone having a lower binder phase content than a nominal binder phase content of the cutting insert, the binder phase content in the outer part of the surface zone being substantially constant. 